50ci is small but your efficiency is still low. you might have very very small leak all over the place. i found out one time that i have very minute leak on 3 different places. only way to find it is remove all electronic except the solenoid and dunk the gun under water with the tank. though make sure that the solenoid plug is dry before you plug it in to the board or else you would messed the board up.

Good luck. Just for kicks and giggles, swap out that o-ring on the front of the bolt too. If it is a leak that only happens when the bolt is in motion, it must be either the sail o-ring, bolt tip o-ring, or the internal breach o-ring I would think.